[libcxx] Fix use of operator comma where the types can be user defined

Summary:
An evil user might overload operator comma. Use a void cast to make sure any user overload is not selected.
Modify all the test iterators to define operator comma. 

Reviewers: danalbert, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5929

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220706 91177308-0d34-0410-b5e6-96231b3b80d8

include/utility

@@ -207,7 +207,7 @@ inline _LIBCPP_INLINE_VISIBILITY
 _ForwardIterator2
 swap_ranges(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2)
 {
-    for(; __first1 != __last1; ++__first1, ++__first2)
+    for(; __first1 != __last1; ++__first1, (void) ++__first2)
         swap(*__first1, *__first2);
     return __first2;
 }